Environmental control systems such as heating, ventilating and air conditioning (HVAC) systems are well known and are designed and implemented to maintain environmental conditions within buildings. A typical installation sees the building divided into zones, and the HVAC system is adapted to maintain each of the zones within predefined environmental parameters (e.g., temperature, humidity, outdoor-recirculated air ratio, etc.). In this exemplary installation, an air distribution system connects each of the zones with an air handling unit (AHU) for providing a supply of conditioned air to the zones.
The AHU generally includes elements for introducing outdoor air into the system and for exhausting air from the system; elements for heating, cooling, filtering and otherwise conditioning the air in the system; and elements for circulating the air within the air distribution system at a desired flow rate. The AHU also includes a controller to control the operation of these elements. A primary task of the AHU is to provide supply air to each of the particular zones to offset the thermal loads imposed on the zone to maintain a comfortable environment for the occupants. Because thermal loads for the zones can vary markedly, it is common for an AHU to be controlled to maintain the supply air temperature at a setpoint value that is sufficiently low to satisfy the zone with the largest load at any given time. If needed, the air stream is throttled and/or reheated at terminal boxes to provide adequate comfort in all zones.
AHU controllers commonly use sequencing logic to determine the most economic way to utilize the elements of the AHU to minimize the cost of maintaining the supply air temperature at the setpoint value. For instance, it is not uncommon for a building subjected to large daily temperature swings to require mechanical heating in the morning and mechanical cooling in the afternoon. Costs associated with mechanical cooling can be reduced using economizer cycle control. Economizer cycle control involves modulating dampers located in the mixing box to control the amount of outdoor air that is introduced to the AHU. Under the proper outdoor air conditions and economizer cycle control, the supply air temperature can be maintained at the setpoint value without the use (or with reduced use) of mechanical cooling.
Economizer cycle control logic typically involves a comparison of the outdoor and return air temperatures or enthalpies. If the outdoor air temperature is greater than some minimum value and less than the return air temperature, an opportunity exists to reduce mechanical cooling costs.
On the surface, sequencing logic and economizer control are quite intuitive. In practice, however, the logic can be very difficult to follow and just as challenging to implement because of numerous exceptional cases (e.g., freeze control, etc.) that must be addressed. This was especially true with pneumatic control systems. It has been demonstrated for pneumatic control systems of actual AHUs that the outdoor air damper and heating coil valve may cycle between fully open and fully closed approximately every two minutes. This wastes energy and leads to excessive component wear. The advent of digital control has done little to improve the situation because, rather than taking full advantage of the programming capabilities of digital controllers, logic used in pneumatic controllers has simply been adapted to the digital controllers.